Engine Misfire

1999 Ford Mustang V6. Out of the blue check engine light came on and engine began to misfire. Hours earlier, I didn't seem to have any problem. Flashing Check engine light indicates engine misfiring and it is defiantly misfiring.

First, I unplugged each wire set (from the ignition coil) individually while the vehicle was idling and all were receiving a steady spark. I assume this means that spark plugs are good and the ignition coil is good? What should I test next?